The rapid development of both communication technology and computer technology has led to the requirement for faster, more sensitive, cheaper semiconductor devices with higher packing densities and more versatile performance. These requirements continue to grow and have resulted in the demand for better materials and better fabrication techniques.
Optical communications technology provides a particularly good example of this trend. Optical communications have been developing at a rapid rate over the last few years. High sensitivity in receivers is desirable because it increases the permitted distance between receivers and reduces the number of repeaters required in a particular communication system. Also of importance in optical communication systems are high speed amplifier systems which permit high bit rates to be transmitted. High speed amplifier systems are also used in other types of circuits including logic circuits, memory circuits, analog circuits, analog switching, high-input-impedance amplifiers, and integrated circuits. Also of interest are such integrated devices as optical detector-amplifier (e.g., PINFET) devices useful in optical communication systems.
One of the most interesting semiconductor compounds for a number of applications is indium phosphide and semiconductor compounds that can be lattice matched to indium phosphide (e.g., indium gallium arsenide, indium gallium arsenide phosphide, indium aluminum arsenide, indium aluminum arsenide phosphide, etc.). There are a number of reasons for this interest in indium phosphide and related compounds. First of all, a variety of interesting ternary and quaternary III-V compounds can be lattice matched to indium phosphide and can be grown as epitaxial layers on indium phosphide. Some of these layers (e.g., indium gallium arsenide phosphide with composition that is lattice matched to indium phosphide) have smaller band gaps than indium phosphide so that in a light emitting diode (LED) structure the indium phosphide is transparent to the emitted radiation. Thus, the output radiation of the LED can be transmitted through an indium phosphide substrate. Other types of layers that can be lattice matched to an indium phosphide substrate have attractive carrier transport properties. For example, indium gallium arsenide of suitable composition can be lattice matched to indium phosphide substrates and has extremely high mobilities. Such device structures provide for the possibility of extremely high speed devices and amplifier devices with high transconductances.
Also, the development of optical communication systems has influenced the importance of various semiconductor materials. For example, with many of the fiber optic materials now available, best performance is achieved in the radiation wavelength region around either 1.55 .mu.m or, alternatively, near 1.3 .mu.m. The quaternary system indium gallium arsenide phosphide can be adjusted in composition so that it is lattice matched to indium phosphide and has a band gap such as to emit radiation in the region of 1.55 .mu.m. Another composition permits lattice matching in the 1.3 .mu.m region. Thus, indium phosphide substrates with good material properties are highly desirable.
It is highly desirable to have a device surface low in defect density. High defect densities in indium phosphide substrates have a number of drawbacks in device fabrication and performance. First of all, epitaxial layers grown on such substrates are of poor quality, generally reproducing the high defect density of the substrate. Also, high defect densities have an adverse affect on etching and metalization processes used in device fabrication, making device fabrication difficult and yields low.
For integrated circuits, the requirements for low defect densities are even more stringent. Here, uniformity of the various units in the integrated circuit is necessary for optimum performance. For example, in integrated circuits with large collections of field effect transistors (FETs), defects in the semiconductor structure cause variations in the electrical properties (e.g., threshold voltage) of the individual FETs. This is highly undesirable in many types of integrated circuits, both analog and digital, such as logic circuits, memory circuits and amplifier circuits. Defects often lead to low yields in circuit fabrication. Also, in circuits such as PINFET structures, defects often affect optical properties as well as electrical properties.
Conventional bulk indium phosphide as well as semi-insulating indium phosphide (e.g., Fe-doped InP) have high defect densities that limit their usefulness in some applications. Both n-type and p-type bulk indium phosphide often have lower defect densities. Particularly good results are obtained for n-type indium phosphide doped with sulfur to a concentration of 1.times.10.sup.17 to 3.5.times.10.sup.19 (or saturation) with the range from 8.times.10.sup.18 to 2.times.10.sup.19 atoms/cm.sup.3 yielding very low defect densities. Although various growth procedures may be used, the liquid-encapsulated Czochralski (LEC) method is generally employed. Also, p-type bulk indium phosphide often has low defect densities with zinc doping in the range 1.times.10.sup.17 to 2.times.10.sup.18 atoms/cm.sup.3 or saturation yielding very low defect densities. The range from 1.times.10.sup.18 to 2.times.10.sup.18 atoms/cm.sup.3 generally yields lowest defect densities. Other growth procedures may also be used including various temperature gradient procedures such as the Bridgeman process, horizontal and vertical gradient freeze methods, etc. A particularly interesting procedure, described in U.S. Pat. No. 4,404,172, issued to William A. Gault on Sept. 13, 1983, also yields low defect densities but at lower doping levels, e.g., 5-20.times.10.sup.17 atoms/cm.sup.3. Often, defect densities less than 100 cm.sup.-2 are obtained by this procedure.
Bulk indium phosphide material that is doped n-type or p-type generally cannot be used directly for substrates in semiconductor devices because the high conductivity prevents sufficient isolation between adjacent circuit elements. (This is not a problem for discrete devices which are cut apart after fabrication.)
A particularly unique solution to this problem is the use of an insulating layer on top of the n-type or p-type indium phosphide substrate. Particularly good results are obtained with epitaxial layers of semi-insulating indium phosphide typically grown by metal organic chemical vapor deposition (MOCVD). Although a variety of dopants can be used to make the indium phosphide semi-insulating, (e.g., Cr, Fe), excellent results are obtained with Fe. Typical concentration ranges are 10.sup.15 to 10.sup.18 atoms/cm.sup.3. This work is described in a number of references including "Growth of Fe-Doped Semi-Insulating InP by MOCVD" by J. A. Long et al. Journal of Crystal Growth, 69, pp. 10-14 (1984); "Electrical Characterization of Fe-Doped Semi-Insulating InP Grown By Metalorganic Chemical Vapor Deposition" by A. T. Macrander et al, Applied Physics Letters, 45, pp. 1297-1298 (1984).
Particularly desirable is a process for making integrated (or discrete) semiconducting circuits on indium phosphide substrates. Preferred is a rapid, inexpensive procedure for making such circuits where packing density can be made high and line design rules very small.